An inflatable airbag restraint system is a safety device for protecting automotive vehicle occupants in a collision. In such a system, an airbag is stowed in an uninflated and folded condition in a covered compartment located on the steering wheel or behind the instrument panel. The open mouth of the airbag is situated around gas outlet ports of a gas generator or inflator used to inflate the airbag. In the event of a collision, an on-board crash sensor, detecting the sudden deceleration of the vehicle indicative of the onset of the collision, immediately sends an electric signal to an ignition system located in the inflator. The ignition system fires and initiates combustion of the gas generant also housed in the inflator, which, upon burning, rapidly produces large volumes of high pressure gas which are directed into a filtering and cooling system and vented into the folded airbag. The airbag is caused to expand and deploy in a matter of milliseconds out of its covered compartment into position in front of the vehicle occupants to effectively cushion the occupants against injury-causing impact with interior structures of the vehicle.
The operational requirements of airbag inflators are very demanding. First, the inflator must remain operative for the life of the vehicle. Next, upon activation, the inflator must produce large volumes of relatively cool, non-toxic, and non-corrosive gas in a matter of 30 to 40 milliseconds in order to inflate and deploy the airbag in a timely fashion for effective occupant cushioning. Many forms of driver's side, solid fuel, airbag inflators have been proposed. Recent emphasis on weight reduction has created the need for lighter weight airbag inflators. U.S. Pat. No. 4,547,342 (Adams et al.) and U.S. Pat. No. 4,561,675 (Adams et al.) disclose a few light weight, three-chambered, aluminum inflators which are currently practiced in the art. A similar inflator is shown in U.S. Pat. No. 5,419,578 (Storey et al.).
Typical prior art airbag inflators have a cylindrical housing or canister with three distinct chambers formed inside an outermost wall in the divided spaces created between two inner cylindrical side walls and the outermost wall. In general, there is a central wall between the central ignition system and the encircling solid gas generant material, and another wall spaced outside the central wall between the gas generating material and the encircling filter assembly. The gas produced must flow through ports in both internal walls before passing through diffusion ports in the outermost wall into the airbag. Each ported internal wall and separate chamber adds complexity, size, weight, thermal and fluid flow inefficiencies, and cost to the inflators.
Adding further complexity to the prior art inflators is a typical two-stage ignition system housed inside the central ignition chamber. In the first stage, an electrically activatable squib is used which is filled with a primary igniter charge, for example, powdered ZrKCIO.sub.4. The squib protrudes in a central ignition chamber from the bottom of the inflator. A pair of leads extend from the squib outside the inflator for connection to the on-board crash sensor circuit. The leads are bridged within the squib by a resistance wire embedded in the primary charge. The second stage uses a igniter cup having a recessed bottom which is filled with a secondary igniter charge, for example, powdered BKNO.sub.3, situated on top of the squib. In operation, the electric current sent from the on-board crash sensor is passed into the squib to ignite the primary charge through resistive heating. Upon ignition, the flame and hot gas produced ignites the secondary igniter charge, bursting the cup and releasing flame and hot gas through the ignition chamber ports into the adjoining combustion chamber for ignition of the gas generant and production of the inflation gas for airbag deployment.
Each ignition stage, however, adds complexity and cost to the inflator. Also, the inflator becomes less reliable, since a number of possible ignition failure points exist along the ignition train. For instance, the igniter charges are susceptible to atmospheric moisture during manufacture and inflator assembly and storage. Moisture contamination can alter their ignition and combustion properties, which could lead to a failure in one or both of the ignition stages and cause a catastrophic inflator misfire. Handling by assembly workers of multiple ignition components might also lead to contamination, dangerous premature firing, and faulty assembly.
Another disadvantage of the two-stage ignition system is that it contributes to the generation of undesirable effluents, such as CO, NH.sub.3 and NO.sub.x, resulting from incomplete and inefficient combustion of the gas generant materials. The central location of the ignition system tends to inhibit uniform and efficient combustion of the gas generant bodies within the combustion chamber. This is because the ignition blast must propagate from a limited number of ignition chamber ports radially outward over the entire gas generant bed. Gas generant material located relatively far way from the ignition chamber ports is ignited at a later time than the material located adjacent those ports. When the generant is ignited in stages in this manner, it results in low pressure burning which is undesirable.
Ignition blast propagation problems are ameliorated somewhat when standard sodium azide gas generants pellets are employed. Sodium azide fuels due to their fast burn rates (i.e., about 0.9-1.0 inch per second) need only be pressed into relatively large sized pellets in order to burn out and produce gas within the desired time for effective deployment of the airbag. With the larger sized pellets packed within the combustion chamber, many voids are created in the generant bed which provide channels through which the ignition blast can rapidly travel.
Recently, it has been proposed to use gas generants with slower burn rates than sodium azide fuels. This would allow the selection of a wider variety of gas generants for use in airbag inflators, some of which have advantageous properties over sodium azide, such as lower toxicity and easier disposal. Such generants must be compacted into much smaller sized bodies in order to burn out at quick enough rates to be effective to inflate the airbag in a timely manner. However, upon loading of such bodies in the combustion chamber, they become tightly packed together, creating minimal voids in the chamber. Consequently, the high packing density tends to block the central ignition blast from traveling radially outward throughout the gas generant bed, thereby making it difficult to uniformly ignite the gas generant bodies. This tends to promote stagewise burning, leading to inefficient combustion, increased effluents, and gas production outside the effective time.
A simpler single-stage direct electrical ignition system for an airbag inflator is disclosed in U.S. Pat. No. 3,606,377 (Martin) wherein a resistance wire is axially embedded along the length of an elongated molded body of a solid pyrotechnic gas generant. The opposite ends of the wire extend from the opposite ends of the gas generant body and are connected to the electrical circuit of the vehicle crash sensor. However, there are problems associated with embedding an igniter wire in a gas generant body. The wire may become damaged when molding the gas generant body around the wire which could go unnoticed during inflator assembly. Damage to the igniter wire would prevent the current sent from the crash sensor circuit from being delivered, and in a collision, the igniter wire would fail to ignite the gas generant, resulting in inflator failure and no inflation and deployment of the airbag, which, in turn, could cause catastrophic injury to the vehicle occupant.
What is needed is a simpler airbag inflator containing an improved ignition system which is easier to assemble, simpler in construction, safer, more reliable and more efficient in operation, and which can be used with smaller bodies of low burn rate gas generants.